Chromosomal Locus for Cadmium Resistance in Pseudomonas putida Consisting of a Cadmium-Transporting ATPase and a MerR Family Response Regulator

Abstract

Pseudomonads from environmental sources vary widely in their sensitivity to cadmium, but the basis for this resistance is largely uncharactarized. A chromosomal fragment encoding cadmium resistance was cloned from Pseudomonas putida 06909, a rhizosphere bacterium, and sequence analysis revealed two divergently transcribed genes, cadA and cadR. CadA was similar to cadmium-transporting ATPases known mostly from grampositive bacteria, and to ZntA, a lead-, zinc-, and cadmium-transporting ATPase from Escherichia coli. CadR was related to the MerR family of response regulators that normally control mercury detoxification in other bacterial systems. A related gene, zntR, regulates zntA in E. coli, but it is not contiguous with zntA in the E. coli genome as cadA and cadR were in P. putida. In addition, unlike ZntA and other CadA homologs, but similar to the predicted product of gene PA3690 in the P. aeruginosa genome, the P. putida CadA sequence had a histidine-rich N-terminal extension. CadR and the product of PA3689 of P. aeruginosa also had histidine-rich Cterminal extensions not found in other MerR family response regulators. Mutational analysis indicated that cadA and cadR are fully responsible for cadmium resistance and partially for zinc resistance. However, unlike zntA, they did not confer significant levels of lead resistance. The cadA promoter was responsive to Cd(II), Pb(II), and Zn(II), while the cadR promoter was only induced by Cd(II). CadR apparently represses its own expression at the transcriptional level. However, CadR apparently does not repress cadA. Homologs of the cadmium-transporting ATPase were detected in many other Pseudomonas species.

Resistance Determinants of Pseudomonas Species from Aquaculture in Australia

“…Instead we detected cadA which is mostly found in Gram-positive bacteria [50][51][52][53]. The genome sequences of several Gram-negative bacteria have revealed homologs of cadA [54,55] and there are some reports of cadA-encoded cadmium resistance being identified in some Pseudomonas species [56,57]. The cadA gene detected in this study is believed to be chromosomally located since southern hybridisation did not detect the presence of the gene on the plasmids in any of the isolates tested.…”

“…The cadA gene detected in this study is believed to be chromosomally located since southern hybridisation did not detect the presence of the gene on the plasmids in any of the isolates tested. Chromosomally located cadA have also been reported in P. putida [57].…”

Improving the sensitivity of bacterial bioreporters for heavy metals

“…The existence of a Zn-specific sensor element is peculiar, as all of the transcription factors and resistance mechanisms known for Zn 2+ thus far cross-react with Cd 2+ and Pb 2+ (see Table 1). 17,[21][22][23][24] However, as only three sensor elements (the promoters of zntA from E. coli, cadA from P. putida and cadA from S. aureus) have been previously used for the construction of Zn/Cd/Pb-bioreporters, it is still possible that an open-minded search for new heavy-metal-regulated promoters would yield bioreporters with different specificities.…”

A novel CPx‐ATPase from the cadmium hyperaccumulator Thlaspi caerulescens

“…Although similar motifs were found in TcHMA4-C and AtHMA4-C, this domain is also the most divergent part between the two sequences. CC dipeptides were only found, together with a His-rich domain, in the N-terminal region of bacterial CPx-ATPases and were associated with Cd 2þ tolerance [28,29]. The function of these putative heavy metal binding motifs remains to be elucidated.…”

“…All these three peptides also contained three additional cysteine motifs -C(x) 4 C, C(x) 3-5 CC, CC -and a His rich domain within their extended C-terminus which could be involved in heavy metal binding. Further examination of the two putative HMA4 COOH tails revealed that the C(x) 3 CC motifs were in a larger cysteine motif, C(x) [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] C(x) 3 CC for Thlaspi and C(x) [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] C(x) 3 CC for Arabidopsis. This sequence is similar to the recently described TRASH domain, C(x) [19][20][21][22] C(x) 3 C, predicted to be involved in heavy metal sensing, trafficking and resistance [25].…”